The first step in manufacturing plasterboard typically resides in manufacturing an aqueous suspension of calcium sulfate hemihydrate. In concrete terms, hemihydrate in the form of a powder is added to water being agitated in a mixer in the presence of a dispersing agent. It becomes hydrated when it contacts the water, forming calcium sulfate dihydrate, meaning gypsum.
The function of the dispersing agent is to ensure that the calcium sulfate hemihydrate is in an even state of dispersion within the aqueous phase; its effectiveness may particularly be measured by the quantity of calcium sulfate hemihydrate that is dispersable in order to obtain a given rheology (or viscosity value). These agents are called “dispersing”, “water-reducing”, and “plastifying” agents, and/or are said to improve the “consistency” or “fluidity” of the gypsum suspension.
Various additives are added in parallel in the aqueous suspension, such as gluing agents, accelerators, etc. It is then mixed with foam taken from a generator (the water is injected into a turbine in the presence of a foaming agent), then sandwiched between two rolling paper strips. The whole article is then cut into the desired dimensions of plasterboard. Regarding the manufacturing of such boards, one may refer to the documents U.S. Pat. No. 5,879,446 and the Kirk-Othmer Encyclopedia of Chemical Technology (Second Edition 1970, vol 21, pp. 621-24, and fourth edition 1992, vol 4, pp. 618-619).
The mixture between the suspension of calcium sulfate hemihydrate and foam only lasts a few seconds. However, as the structure of the foam is particularly sensitive to agitations in the environment, it often degrades during this step. Such degradation leads to the formation of large bubbles which affect the adhesion of the suspension of calcium sulfate hemihydrate onto the strips of paper; it is therefore necessary to overdose the quantity of foaming agent—which accordingly decreases the productivity of the method—in order to obtain the desired quantity of foam. This issue is recounted clearly in the documents JP 08-217505 and U.S. Pat. No. 6,264,739.
The foam is also sensitive to the nature of the dispersing agent. The person skilled in the art initially used lignosulfate and naphtalene-sulfonate products, but their effectiveness at dispersing quickly proved insufficient. So-called “comb” dispersing agents were then engineered, made of a (meth)acrylic skeleton onto which are grafted alcoxy- or hydroxy-polyalkylene glycol lateral chains.
Examples of such dispersing agents are found in the patents JP 08-217505 and U.S. Pat. No. 6,264,739, both incorporated herein by reference, cited above. These documents deal with the question of interactions between the foam and the dispersing agent, by way of tests based on the mixture of foam and a suspension of calcium sulfate hemihydrate containing a dispersing agent. This mixture is produced during agitation for a quarter hour. It is therefore a “close” mixture between the components, created for a time which is very long compared to the few seconds in industrial-scale operations.
The results obtained according to these two patents appeared to the inventors to demonstrate that naphtalene-sulfonate dispersing agents are much more destructive to foam than their comb counterparts. However, the structure of the foam is altered by the very nature of the method that is used: the 15 minutes of mixing necessarily destructure the foam, and it was reasonable to question just how representative the results obtained were.